1. Field of the Invention
The present invention relates to a wireless communications system and a wireless communications device, and more particularly to a close proximity wireless communications system that focuses on the importance of bandwidth protection between wireless communications devices, and a wireless communications device that forms a part of the close proximity wireless communications system.
2. Description of the Related Art
In known wireless communications systems, there are occasions when electric power is needlessly consumed even when data transfer does not have to be performed. For example, in an autonomous distributed wireless communications system, each node must periodically transmit a beacon signal to notify its presence to other surrounding nodes. Alternatively, in a centrally controlled wireless communications system, each node must periodically respond to the central control node to notify its presence. In these situations, even if data transfer is not performed, electric power is wastefully consumed.
With regard to methods used for data transfer, in Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) access control that is typically used in 802.11a/b/g standards, a random back-off method is used as a way of evenly distributing bandwidth to each node within the wireless service area (for example, refer to Japanese Patent Application Publication No. JP-2006-222608). In the random back-off method, a random back-off is selected in each node before transmission, thereby allowing even use of bandwidth. Thus, it is possible to inhibit any particular node from monopolizing bandwidth.
FIG. 11 illustrates a concrete example of a known wireless communications system and shows devices #1 to #4 and their wireless communication ranges. This wireless communications system is envisaged for use in, for example, a wireless sensor module, and devices #1 to #4 could be imagined to be, for example, wireless sensor modules located in different rooms within a house. In this wireless sensor system, data is only transferred between the wireless sensor modules when the sensors detect a change.
FIG. 12 is an explanatory figure showing the transfer of data (temperature, humidity, vibration, brightness or the like) between the wireless sensor modules when the sensors detect a change in the wireless sensor system shown in FIG. 11. In FIG. 12, Tx represents transmitted data, and Rx represents received data. As can be seen from FIGS. 11 and 12, devices #1 and #2 are within each others communication ranges and communicate with each other, and devices #3 and #4 are within each others communication ranges and communicate with each other. Thus, the system is configured such that mutual interference does not occur.